Bug#980314: leaflet-markercluster: switch to @rollup/plugin-json

[Pirate Praveen]

Package: leaflet-markercluster
Version: 1.4.1~dfsg-9
Severity: important
As tracked in
https://wiki.debian.org/Javascript/Nodejs/Transitions/Rollup-plugin-json-legacy-rm

and notified in 
https://alioth-lists.debian.net/pipermail/pkg-javascript-devel/2021-January/050132.html

It'd be good to drop the embedded copy of legacy rollup-plugin-json 
component from bullseye.